1. Technical Field of the Invention
The present invention relates to processors and, more particularly, to creation and management of threads in a processor.
2. Background Art
Current superscaler processors, such as a microprocessor, perform techniques such as branch prediction and out-of-order execution to enhance performance. Processors having out-of-order execution pipelines execute certain instructions in a different order than the order in which the instructions were fetched and decoded. Instructions may be executed out of order with respect to instructions for which there are not dependencies. Out-of-order execution increases processor performance by preventing execution units from being idle merely because of program instruction order. Instruction results are reordered after execution.
The task of handling data dependencies is simplified by restricting instruction decode to being in-order. The processors may then identify how data flows from one instruction to subsequent instructions through registers. To ensure program correctness, registers are renamed and instructions wait in reservation stations until their input operands are generated, at which time they are issued to the appropriate functional units for execution. The register renamer, reservation stations, and related mechanisms link instructions having dependencies together so that a dependent instruction is not executed before the instruction on which it depends. Accordingly, such processors are limited by in-order fetch and decode.
When the instruction from the instruction cache misses or a branch is mispredicted, the processors have either to wait until the instruction block is fetched from the higher level cache or memory, or until the mispredicted branch is resolved, and the execution of the false path is reset. The result of such behavior is that independent instructions before and after instruction cache misses and mispredicted branches cannot be executed in parallel, although it may be correct to do so.
Multithreading processors such as shared resource multithreading processors and on-chip multiprocessor (MP) processors have the capability to process and execute multiple threads concurrently. The threads that these processors process and execute are independent of each other. For example, the threads are either from completely independent programs or are from the same program but are specially compiled to create threads without dependencies between threads. However, these processors do not have the ability to concurrently execute different threads from the same program that may have dependencies. The usefulness of the multithreading processors is thereby limited.
Accordingly, there is a need for multithreading processors that have the ability to concurrently execute different threads from the same program where there may be dependencies among the threads.